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Re: [PATCH v2] failover: specify an alternate MAC address

From: Michael S. Tsirkin
Subject: Re: [PATCH v2] failover: specify an alternate MAC address
Date: Tue, 2 Nov 2021 04:45:59 -0400

On Tue, Nov 02, 2021 at 09:14:51AM +0100, Laurent Vivier wrote:
> On 01/11/2021 10:39, Michael S. Tsirkin wrote:
> > On Wed, Oct 27, 2021 at 11:59:45AM +0200, Laurent Vivier wrote:
> > > If the guest driver doesn't support the STANDBY feature, by default
> > > we keep the virtio-net device and don't hotplug the VFIO device,
> > > but in some cases, user can prefer to use the VFIO device rather
> > > than the virtio-net one. We can't unplug the virtio-net device
> > > (because on migration it is expected on the destination side) but
> > > we can keep both interfaces if the MAC addresses are different
> > > (to have the same MAC address can cause kernel crash with old
> > > kernel). The VFIO device will be unplugged before the migration
> > > like in the normal failover migration but without a failover device.
> > > 
> > > This patch adds a new property to the virtio-net device:
> > > "failover-legacy-mac"
> > > 
> > > If an alternate MAC address is provided with "failover-legacy-mac" and
> > > the STANDBY feature is not supported, both interfaces are plugged
> > > but the standby interface (virtio-net) MAC address is set to the
> > > value provided by the "failover-legacy-mac" parameter.
> > > 
> > > If the STANDBY feature is supported by guest and QEMU, the virtio-net
> > > failover acts as usual.
> > > 
> > > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > 
> > Wait a second. What if config is read before features are set?
> > Are we going to provide a legacy or a new mac?
> We provide the new MAC and at this point the primary device is not plugged.
> When features are set:
> - if STANDBY is set, the primary device is plugged, and secondary
> (virtio-net) uses the new MAC
> - if STANDBY is not set:
>     - if legacy MAC is provided:
>         the primary device is plugged and legacy MAC is used
>     - else
>         the primary device is not plugged and new MAC is used.
> > I guess current guests do not do this, but the spec does allow this,
> > and then the mac will apparently change for the guests.
> What I read in virtio 1.0 specs, "3.1.1 Driver requirements: Device
> initialization", is the virtio configuration space (step 7) is is accessed
> after the features are negotiated. I don't think the part in step 4 can
> involve the MAC address, and moreover the config is not read before, but
> during the negotiation (I guess we can see that as the config access is part
> of the negotiation).
> 3.1.1 Driver Requirements: Device Initialization
> The driver MUST follow this sequence to initialize a device:
> 1. Reset the device.
> 2. Set the ACKNOWLEDGE status bit: the guest OS has notice the device.
> 3. Set the DRIVER status bit: the guest OS knows how to drive the device.
> 4. Read device feature bits, and write the subset of feature bits understood
> by the OS and driver to the device. During this step the driver MAY read
> (but MUST NOT write) the device-specific configuration fields to check that
> it can support the device before accepting it.
> 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature
> bits after this step.
> 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> otherwise, the device does not support our subset of features and the device
> is unusable.
> 7. Perform device-specific setup, including discovery of virtqueues for the
> device, optional per-bus setup, reading and possibly writing the device’s
> virtio configuration space, and population of virtqueues.
> 8. Set the DRIVER_OK status bit. At this point the device is “live”.


Device Requirements: Device Configuration Space
The device MUST allow reading of any device-specific configuration field before 
FEATURES_OK is set by the
driver. This includes fields which are conditional on feature bits, as long as 
those feature bits are offered by the

now, we can maybe make an exception for mac.
But this is something to discuss on the virtio TC mailing list.

> > 
> > It might be cleaner to just add a PRIMARY_MAC feature -
> > would need guest work though ...
> We can't add a new feature: the goal of this patch is to be able to use the
> primary device (VFIO) with kernel that doesn't support STANDBY feature. If
> we can add a feature, to add the STANDBY feature would be a better choice.
> If changing the MAC address is not acceptable we can return to a mix of v1 
> and v2 of my patch:
> "virtio: failover: allow to keep the VFIO device rather than the virtio-net 
> one"
> https://patchew.org/QEMU/20210729191910.317114-1-lvivier@redhat.com/
> that disables the virtio-net driver on the module probe.
> Thanks,
> Laurent

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